Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 491

Fig. 10.24 Variations of parameters of power provision (Plant A, promising geological
conditions; for reasons of comparison also Plant B is shown varying the credits for heat)
Environmental analysis. In the following section the geothermal power genera-
tion is assessed according to selected environmental effects. Again, assessment is
performed with regard to the effects caused during construction of the plant, op-
eration, malfunctions, and the end of operation.
Construction. Environmental effects can already arise during exploration and
drilling of wells to unlock the geothermal energy. Effects are comparable to those
of crude oil and natural gas exploration. However, besides noise effects which
have to be within the limits defined by the existing legal framework, only very
few environmental effects remain once drilling is completed, as it is possible to
re-cultivate the drill site except for the well head. Furthermore, such drilling ef-
fects on the environment are limited to the short period of drilling only, which,
depending on the respective depth, usually takes 3 to 6 months.
Normal operation. Due to the circulation of the geothermal fluid within the heat
exchanger (i.e. within the underground), small concentrations of salts and min-
erals may be dissolved several kilometres below the earth's surface. Outside of
volcanic areas, heavy metal or sulphurous compounds are either not available, or
their concentrations are so low that they are hardly detectable. However, these
substances do not have a detrimental effect on the environment, as the geothermal
fluid is transferred within a closed circuit and dissolved substances are subse-
quently retransferred into the underground /10-1/.
If the underground is cooled off due to the re-injection of cooled geothermal
fluid the mechanical stress field may change, which may generate local relief and
thus lead to microseism or small earthquakes. However, during experiments per-
formed at Soultz-sous-Forêts microseism has to date only be observed during
extensive stimulations and thus during the production of the heat exchanger